Field of the Invention
The present invention relates to a method of and an apparatus for combusting sulfurous fuel in a circulating fluidized bed boiler. The invention especially relates to reducing sulfur dioxide (SO2) content in the exhaust gas emanating from a circulating fluidized bed (CFB) boiler by using a dry circulating fluidized bed (CFB) scrubber.
Description of the Related Art
When combusting sulfur-containing fuels, the sulfur in the fuel is converted to sulfur oxides, mainly, sulfur dioxide, which may be harmful if emitted to the environment in large quantities. Dry flue gas scrubbing in a circulating fluidized bed scrubber is a commonly used method to reduce SO2 content in the flue gases emanating from different kinds of plants, such as power boilers, cement kilns, glass furnaces, and waste incinerators. When using a circulating fluidized bed (CFB) boiler, SO2 content of the exhaust gas is commonly reduced by feeding limestone into the furnace of the boiler. At the temperatures prevailing in the furnace of a CFB boiler, usually from 750° C. to 950° C., calcium carbonate (CaCO3) of the limestone is calcined to form calcium oxide (CaO), which reacts with sulfur oxides to produce calcium sulfate (CaSO4), which can be removed from the furnace along with the ashes produced in the combustion.
Relatively good sulfur-reduction efficiency can be obtained in a CFB boiler solely by feeding limestone directly into the furnace. However, because a layer of CaSO4 forming on the surface of the CaO particles prevents the core of the particles from reacting with SO2, an excess of limestone has to be fed into the furnace in order to obtain a desired level of SO2 removal. For example, in order to achieve 98% sulfur reduction efficiency in the furnace, a Ca/S molar ratio as high as four to five is required.
Ash formed in a combustion process can be divided to fly ash, which is entrained with the exhaust gas from the furnace and bottom ash that collects to the bottom of the furnace. During the combustion, bottom ash is formed at a rate which, as well as the composition of the bottom ash, depends on the operating conditions of the boiler, especially, on the properties of the fuel, sorbent, and inert bed material fed into the furnace. Typically, from 10% to 50% of the ashes formed in a circulating fluidized bed boiler are removed from the furnace as bottom ash. Bottom ash is generally coarser than the fly ash.
When using high Ca/S ratios, such as from four to five, the bottom ash and fly ash discharged from the furnace invariably contain a large amount of CaO, typically, from about 10% to more than 50%, which makes the use or disposal of the ashes difficult.
Another problem associated with the conventional sulfur-reduction process in a CFB furnace is that the calcination of calcium carbonate is an endothermic reaction, with a reaction energy of 178.4 kJ/kmol. Thus, the calcination of excessive amounts of limestone decreases the thermal efficiency of the boiler. For example, in order to achieve 98% sulfur reduction when combusting coal containing 2% sulfur, limestone is introduced at a rate providing a Ca/S ratio of about five, and the energy required for calcination reduces the thermal efficiency of the boiler by about 2 percentage points.
In order to eliminate the need for such high Ca/S ratios, U.S. Pat. No. 4,185,080 teaches to remove ashes, calcium oxide, and calcium sulfate from a fluidized bed and to separate fine grade ashes, e.g., by sifting, and calcium sulfate, e.g., in a drum, from the calcium oxide. The calcium oxide is then dry slaked to an extremely fine-grained calcium hydroxide that is blasted into the fluidized bed or into a free board above the bed to react with sulfur oxides in the combustion gases. U.S. Pat. No. 5,345,883 teaches to re-activate recirculating sorbent material of a circulating fluidized bed boiler by injecting high pressure water as a liquid or steam to mechanically fracture used sorbent particles so as to expose unreacted sorbent material in the core of the particles. U.S. Pat. No. 5,551,357 and U.S. Pat. No. 6,912,962 show further methods of enhancing flue gas desulfurization by, e.g., hydrating a portion of CaO in the bottom ash before injecting it back to the furnace.
An alternative for enhancing sulfur reduction in a fluidized bed boiler is to perform only a portion of the required sulfur dioxide reduction in the furnace, and to arrange a second sulfur dioxide reduction device downstream of the furnace. U.S. Pat. No. 7,427,384 suggests feeding calcium carbonate to the furnace of a circulating fluidized bed boiler so as to provide a Ca/S molar ratio of at most about 1.0 and further reducing the sulfur content of the flue gas in a sulfur reducing stage, such as dry sulfur-reduction equipment, downstream of the furnace. Correspondingly, U.S. Pat. No. 7,862,789 suggests conveying flue gases from a circulating fluidized bed boiler to a flash dry absorber and introducing lime into the flash dry absorber.
U.S. Pat. No. 4,309,393 discloses a sulfur-reduction method for a fluidized bed boiler, in which limestone is added to the furnace in a Ca/S ratio ranging from 1 to 1.5, so as to provide sulfur reduction of 30% to 60% in the furnace. CaO-containing ashes are then collected from the furnace and slaked to form lime slurry for utilization in an SO2 stripping unit, such as a spray dryer, disposed in the flue gas duct downstream of the furnace. U.S. Pat. No. 6,290,921 teaches to feed sieved CaO-containing bottom ash of a circulating fluidized bed boiler to a moistening device between a dust separator and a contact reactor of a sulfur reduction system.
An object of the present invention is to provide a method of and an apparatus for combusting sulfurous fuel in a circulating fluidized bed boiler with improved desulfurization.